Surface modification employs different chemical reactions to produce improved properties and structures of the materials. Nitridation is a process to form nitrides on metal substrates, usually a hard surface of metal nitrides. This process has been widely used to improve mechanical properties and corrosion resistance of iron and steels. However, due to the kinetics of the nitride formation, the nitridation process normally operates at high temperature (typically over 550° C.) for a long period of time (about 8 to 16 hours), which may degrade the substrate materials. The enhancement of plasma source and ion implantation can decrease the operating temperature to about 300 to about 400° C. for stainless steels. Nitridation has also been made by ion implantation at room temperature with doses on the order of about 2.5×1016 to about 2.0×1017 N2+/cm2. Although these methods modify the steels surface, significant drawbacks are the high cost associated with the vacuum equipment and high temperature needed for promoting the chemical gas reaction.
Stainless steels have good mechanical strength, high chemical stability, are suitable for mass production, offer a wide range of choices, and at relatively low cost. In these respects, the material is widely accepted as a polymer electrolyte membrane fuel cell (PEMFC) bipolar plate candidate. Ferrite and duplex stainless steels are available at low cost and stainless steels such as AISI446 and 2205 steels are also good candidates for PEMFC bipolar plates. The American Iron and Steel Institute (AISI) naming system is one of the most widely accepted systems for designating the various compositions of steels. AISI is a not-for-profit trade association that serves as the voice of the North American steel industry in the public policy arena and advances the case for steel in the marketplace as the material of choice.
The drawback of using stainless steel bipolar plates is the higher interfacial contact resistances due to surface oxide films. These surface films provide protection for the base material from corrosion in PEMFC environments. However, the high resistance of the film also decreases the surface electrical conductivity and renders the stainless steel unusable. Thermally nitrided AISI446 steel may be used in which a discontinuous mixture of nitrides and oxide is formed that provide excellent interfacial conductivity while maintaining good corrosion resistance of the metal. Nitrogen is incorporated into the naturally occurring oxide layer on the surface of the metal. However, the processing and associated costs of high temperature during thermal nitridation represents significant drawbacks. Consequently, there is a need for a low-cost surface nitridation process for stainless steel which exhibits properties similar to thermally nitrided steels.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.